The present invention relates to oxidation resistant carbon/carbon composites and pertains particularly to improved oxidation resistant sealants for such composites.
Materials having high strength to weight ratios are in great need for operation under severe conditions in ranges of temperatures in the thousands of degrees F. Jet engines for example now operate at temperatures of several thousands of degrees F., and are expected to operate at these temperatures with high reliability and minimum maintenance. Materials used in these applications must be capable of withstanding corrosion and erosion at elevated temperatures for long periods of time. They must be hard, tough and wear resistant at these elevated temperatures.
Carbon/carbon composites have been known for some time, and are desirable in applications wherein high strength to weight ratio at elevated temperatures is desired. Such composites have been developed in recent years with oxidation resistant coatings for use in high temperature environments. Carbon/carbon composites typically include a carbonous matrix formed by carbonization of a carbon char-yielding material. Carbon or other fibers are typically incorporated in the a carbon matrix so as to reinforce the same. These materials are formed in many ways, for example by impregnating graphite cloth with a carbon char-yielding material, and then carbonizing the impregnate through the application of heat and pressure.
Prior carbon/carbon composites have utilized an in-depth particulate addition to the carbon matrix, with an external refractory coating to prevent oxidation of the composite at high temperatures. The particulates consist of metal alloy or metal carbide powders which do not appreciably react to the carbon fibers or matrix. The external coating forms a primary oxygen barrier and is oxidation resistant. Examples of these coatings are silicon carbide (SiC) or silicon nitride (Si.sub.3 N.sub.4). These coatings are usually applied by chemical vapor deposition (CVD). However, due to the inherently low coefficient of thermal expansion of the carbon/carbon and the elevated coating deposition temperatures, the CVD coating becomes microcracked. At temperatures above which carbon oxidizes (500 degree C.), these microcracks serve as paths for oxygen ingress, allowing the carbon fibers and matrix to oxidize.
If the coating could be made crack free, either by raising the expansion coefficient of the substrate, or lowering the expansion coefficient of the coating, the matrix and fibers would be protected from oxidation. Alternatively, filling the microcracks in the carbon/carbon coating with a viscous sealant material which will flow, accommodating changes in the crack dimension with temperature, will also protect the matrix and fibers from oxidation. Attempts at this latter solution have been proposed in the past. In my co-pending application Ser. No. 805,887, filed Dec. 6, 1985, I disclose "Oxidation-Inhibited Carbonous Materials" that utilize a binary system of boron and a metal or metalloid which oxidizes at high temperatures to form a sealant of boron oxide-based glass. The metal or metalloid oxide is partially soluble in boron oxide and stabilizes the viscosity of the boron oxide at high temperatures. Prior attempts to solve the oxidation problem by sealing the microcracks have not been completely satisfactory at certain temperatures.